Endobacteria in the diatom Pinnularia (Bacillariophyceae).: I.: Scattered ct-nucleoids explained:: DAPI-DNA complexes stem from exoplastidial bacteria boring into the chloroplasts

A local strain of the pennate diatom Pinnularia cf. nobilis was investigated using cytochemistry and fluorescence and EM techniques. The regular perforation of the chloroplasts of P. nobilis and the lack of a typical diatom pyrenoid were confirmed at the ultrastructural level. Cavities and channels in the complex secondary plastid were found to harbor symbiotic bacteria, and their DNA elicited DAPI fluorescence. Wheat germ agglutinin, labeling bacteria walls, elicited a similar fluorescence pattern. Previous speculation that the apochlorotic DNA-positive dots in the plastids of several Pinnularia species are scattered ct-nucleoids is thus refuted. Bacteria were rod shaped and gram negative. They resided in the lumen of the endoplasmic reticulum (ER) during host interphase confined to the specialized ER compartment housing the secondary plastid, that is, to the space between the third and the fourth membrane profile, encircling the chloroplast. TEM images of chemically and cryofixed cells revealed that cavities resulted from the interaction of bacteria with the plastid according to the following sequence, alignment, attachment, deformation, and disintegration. This occurred without visible injury to the primary chloroplast envelope or the relict cell membrane of the reduced ancestral red alga that surrounds the chloroplast. The patterned arrangement of bacteria suggests recognition sites on the vestigial cell membrane, thought to interact with surface groups on the bacteria. The intimate association between bacteria and secondary plastid inside the common specialized ER cisterna suggests they form a functional unit. Comparison of thylakoid profiles, disrupted by bacteria in Pinnularia , with those disrupted by the pyrenoid in other pennate diatoms (e.g. Trachyneis ) revealed a significant ultrastructural resemblance. No aposymbiotic Pinnularia cells were found at the sampling site.